Peculiarities of Activation of the Shoulder Belt and Shoulder Muscles in Generation of Different-Direction Isometric Efforts by the Forearm |
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| Authors: | I V Vereshchaka A V Gorkovenko V I Khorevin A N Tal’nov V V Korneyev A I Kostyukov |
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| Institution: | (1) Department of Physiology and Pharmacology, The University of Western Ontario, Medical Science Bldg, London, ON, N6A 5C1, Canada;(2) Department of Psychology, The University of Western Ontario, Social Sciences Bldg, London, ON, N6A 5C2, Canada; |
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| Abstract: | We studied central motor commands, CMCs, coming to the muscles that flex and extend the shoulder and elbow joints in the course
of generation of voluntary isometric efforts of different directions by the forearm; the efforts were initiated according
to a visual signal. Amplitudes of EMGs recorded from the muscles of the shoulder belt and shoulder and subjected to full-wave
rectification and low-frequency filtration were considered correlates of the CMC intensity. An effort of the preset direction
was developed within the operational space of the horizontal plane with angles 30 deg in the shoulder joint (external angle
with respect to the frontal plane) and 90 deg in the elbow joint. We plotted sector diagrams of the logarithmic coefficient
of the intensity increment of EMGs of the above muscles for the entire set of directions of generated efforts with a 15- or
20-deg step. Orientations of the maxima of EMG activity of the given muscles were rather close to the directions of the maxima
of the force moments generated by these muscles. In most cases, a shift of the direction by one gradation with respect to
the EMG maximum in the respective muscle resulted in a significant decrease in the level of EMG activity. It is shown that
preferential activation of the muscles agonistic with respect to the examined direction of the generated effort was, as a
rule, accompanied by coactivation of the antagonist muscles. When “two-joint” isometric efforts are formed, realization of
the socalled synergic muscle tasks (where prevailing contractions of the muscles of the same functional direction for both
joints coincide, i.e., flexion-flexion or extension-extension) is organized in a simpler manner. The programs of “nonsynergic”
contractions (flexion of one joint and extension of another one, or vice versa) are more complex. In different subjects, considerably dissimilar patterns of EMG activity in muscles influencing these joints
could be observed. |
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